Minimal impact of nitrogen addition on bacterial and fungal communities during fungal necromass decomposition in a subalpine coniferous plantation

Fungal necromass is a vital source of stable carbon and available nitrogen in soils, especially in coniferous forests that are extensively colonized by ectomycorrhizal fungi. Here, we examined changes in the necromass microbial community during a 20-week incubation experiment in a subalpine coniferous plantation. We also explored how the necromass microbial community responded to simulated nitrogen deposition, a global change factor that substantially affects nitrogen-limited coniferous forests. The results showed that necromass microbial community was more dominated by copiotrophic bacteria with high community average rRNA operon copy number (3.1-6.4) and fast-growing fungi (moulds and yeasts) than surrounding soils. Ectomycorrhizal fungi also constituted a significant portion of the fungal community during the fast decay stage (first 5 weeks) of necromass decomposition, indicating that fungal necromass may serve as a nitrogen source for the nitrogen-limited trees. The shifts in microbial functional compositions (e.g., decreased proportions of copiotrophic bacteria) as decomposition progressed were partly due to alterations in substrate chemistry (i.e., the levels of aliphatic compounds including soluble carbohydrate, amorphous glucan polymers, and crystalline glucans and chitin). In contrast, nitrogen addition had limited influences on microbial diversity and composition, likely due to the nitrogen-rich nature of the fungal necromass. Overall, our findings reinforce the existence of a specific and dynamic core necrobiome in fungal necromass and enhance our understanding of how fungal necrobiome responds to environmental changes.